1. Field of the Invention
This invention relates to a metal vapor laser device.
2. Description of the Prior Art
A metal vapor laser device generates laser oscillations making use of vapor of a working metal which is enclosed in a laser tube together with a carrier gas.
An exemplary one of metal vapor laser devices which are practically used at present is a so-called positive column type metal vapor laser device which generates laser oscillations making use of a positive column portion of a discharge. More particularly, a positive column type He-Cd laser device is known wherein helium is used as a carrier gas and cadmium is used as a working metal.
Such a He-Cd laser device can continuously oscillate, for example, an ultraviolet ray having a wavelength of 325 nm as well as a visible ray having a wavelength of 442 nm in a short wavelength zone. Due to the characteristic, the demand for such He-Cd lasers has increased in recent years, and He-Cd laser devices are used in various fields as light sources, for example, for laser printers, holography, photo-plotters, color scanners and so on.
In such a He-Cd laser device, however, when cadmium vapor is condensed on an inner wall of a laser tube, helium gas is trapped in the condensed cadmium so that the pressure of the helium gas in the laser tube decreases. Consequently, the output laser power of the laser device decreases, which decreases the life of the laser device.
In order to prevent this, a helium gas supplier is conventionally connected to the He-Cd laser tube to supply helium gas suitably into the He-Cd laser tube.
An exemplary one of such conventional helium gas suppliers includes an outer tube substantially impermeable to helium gas, a partition having a helium gas permeability which varies according to temperature and partitioning the inside of the outer tube into a tank section and a reservoir section which communicates with the inside of a laser tube, and a heater for controlling the temperature of the partition. A pressure detector for detecting the pressure of helium gas is connected to the reservoir section of the outer tube of the helium gas supplier. If the pressure detector detects a decrease of the pressure of helium gas, the temperature of the partition is regulated by the heater to control the permeability of the partition to helium gas.
A Pirani sensor which is one of electric detecting means is conventionally empmloyed as such a pressure detector.
Where an electric pressure detector such as a Pirani sensor is used, however, a discharge readily appears between the anode of the He-Cd laser tube and a detecting element of the electric pressure detector to which a voltage is applied because the detecting element of the electric pressure detector is disposed in a spacing which communicates with a spacing within the helium gas supplier. Accordingly, it is a problem that a laser beam is not oscillated stably in the He-Cd laser device.